diff --git a/src/main/java/com/android/tools/r8/ir/conversion/LensCodeRewriter.java b/src/main/java/com/android/tools/r8/ir/conversion/LensCodeRewriter.java
index 1877053..e9b789a 100644
--- a/src/main/java/com/android/tools/r8/ir/conversion/LensCodeRewriter.java
+++ b/src/main/java/com/android/tools/r8/ir/conversion/LensCodeRewriter.java
@@ -39,17 +39,17 @@
 import com.android.tools.r8.ir.code.StaticPut;
 import com.android.tools.r8.ir.code.Value;
 import java.util.ArrayList;
-import java.util.HashMap;
 import java.util.List;
 import java.util.ListIterator;
 import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
 
 public class LensCodeRewriter {
 
   private final GraphLense graphLense;
   private final AppInfoWithSubtyping appInfo;
 
-  private final Map<DexProto, DexProto> protoFixupCache = new HashMap<>();
+  private final Map<DexProto, DexProto> protoFixupCache = new ConcurrentHashMap<>();
 
   public LensCodeRewriter(GraphLense graphLense, AppInfoWithSubtyping appInfo) {
     this.graphLense = graphLense;